The phrase "for grins" is an informal idiom used to express doing something just for fun or entertainment, without any serious intention or motive. The antonyms of "for grins" would be phrases that suggest seriousness, purpose, and practicality. Some of the potential antonyms of "for grins" could be "for a reason", "with intention", "for a purpose", "with a goal in mind", "with a serious intent", "for a practical outcome", "with a sense of responsibility", etc. These antonyms represent the opposite end of the spectrum to "for grins" and imply that actions are taken with a more significant aim and objective in mind.